www.ejbps.com IV
APPLICATIONS OF BIOSTATISTICS AND EVOLUTION PREFACE
The book is written in simple language so that the students can easily grasp the matter. Some important terms has been incorporated. So that the students may search the useful related for competitive examinations. In the recent years included in the syllabus of almost all Indian Universities in various subjects of Biology or Life Sciences as an independent evergreen subject. Exponential growth in many areas of basic fundamentals made it necessary in some cases to write several chapters on the same topic which was covered in a single chapter in the earlier book. Similarly, in the present volume, separate new chapters have been written on topics which in the earlier title either did not figure at all or were each covered very briefly as a part of a chapter. In the present book, for instance in separate chapters have been written on new topics. The students of Biology at the post graduate (P.G.) under graduate (U.G.) levels needed to the recent Global changes and developments.
Several of my students in the laboratory helped me either in writing some of the chapters or in preparing the list of references and appendix given at the end of this volume. There may also be errors and omissions of technical nature, since in a vast and fast expanding subject like Botany Biotechnology; one cannot claim to have known everything, despite his best efforts.
I am thankful to Department Head, BOS, Staff and Research Scholars (Botany) my family members, inspiration and cooperation my wife and children’s (M. Hamsini) Teachers, Friends, Students and Well-Wishers (Dursheti Sai Charan, M.B.A., Certified Microsoft Office and Windows Specialist). I hope that this book will be useful to students in Life Sciences.
Dr. Mandaloju Venkateshwarlu
CONTENTS TOPICS
APPLICATIONS OF EVOLUTION AND BIOSTATISTICS
Chapter 1 Evolution 1-65
www.ejbps.com 1 CHAPTER I
EVOLUTION 1. INTRODUCTION
The idea that the present forms of life have arisen from the earlier, simpler ones was far from new when Charles Darwin published his book “The Origin of Species” in 1859. The development of the idea of organic evolution through pre-Darwininan, Darwinian, and post-Darwinian eras can be studied under the following periods. The oldest speculations about evolution are found in the classical mythologies of different centres of ancient cultures, such as India, Chinna, Babylong and Egypt. For instance, the classical Indian (Hindu) mythology has forwarded the idea that universe and the body of all living beings are made up of five elements or “Panch Bhota”, namely kshiti (earth), ap (water), teja (fire), marut (air) and vyoum (void orsky). Further, the Padam Puran conceived ten incarnations or “Dashvatarahs” for explaining the evolution of vertebrates. The sequence of “Avatarahs, includes Matsya, fish-like appearance; Kurma, reptile-like; „Varaha‟, mammal-like; „Narasimha‟, half-man and half-lion; “Vamana”, the dwarf man-like appearance (anthropoid); “Parshurama”,a Homa saplens of Pleistocene; “Shri Rama”, “Balrama”, “Shri Krishna” and “Kalpi” – the latter four incarnations depict the formation of clans and communities.
Similar to ancient Indians, the ancient Chinese conceived the world at the beginning as made up of five primary elements – water, wood, fire, soil, and the precious metal, gold. According to their mythology, the positive and negative interactions of these primary five elements were responsible for origin of life and different kinds of adaptations and diversities of organism or “things” were originated from a single source through gradual unfolding and branching.
www.ejbps.com 2 During this period, because the speculation on evolution idea was much shrouded in mystery and mythology, so, this period has come to be known as the period of obscurity.
2. Period of Ancient Greeks and Romans
The classical writings of certain Greek philosophers contain the vague ideas and speculations about the organic evolution. Thus, Thales (624-548 B.C.) suggested aquatic origin of life, i.e., all life originated from oceans. Anaximander (611-547 B.C.) explained the origin of universe. According to him, living things came from primordial fluid and men were first formed as fishes; eventually they cast off their fish skin and took up life on dry land. Anaximenes (382-524 B.C.) believed that life came from air and he proposed abiogenesis. Xenophanes (576-480 B.C.) is credited with being the first person to recognize that fossils, such as petrified shells embedded in rocks, represent the remains of animals that once lived. He also realized that the presence of fossils of marine animals on what is now dry land indicates that the sea once covered the area. He believed that in ancient times earth was covered with water where life began. Empedocles (495-435 B.C.) is considered as the “Father of evolution idea” by many evolutionists for his ideas which he forwarded in his “Poem of Nature”. According to him, life arose spontaneously but gradually and change (evolution) would result when twin forces, the attraction and repulsion, interact with each other. Based on it, he further explained that first living things were cast out in the form of separate parts and by chance these parts were united by attraction force and as a result plants and animals were born. The sex originated later and through reproduction the developing embryo resulted from both paternal and maternal substances and in that fashion distinct species were formed. He believed that when reproduction was established, earth felt no need of special creation of organism. According to Empedocles plants came earlier than animals and perfect forms replaced imperfect types.
www.ejbps.com 3 or sensibility. Within the animal groups he constructed a genetic series leading from the lowest forms up to man placed at the apex. ………is considered as the ……….from this taxonomic fact, he drew thecorrect inference that one higher group evolved from other lower one in the taxonomic series. However, he had the metaphysical belief that the gradual evolution of living things occurred because nature strives to change from the simple and imperfect to the more complex and perfect. An evolutionary explanation of the origin of plants and animals was given by the Roman poet ………….. (99-55 B.C.) in his poem De Rerum Natura.
3. Pre-Darwinian Period
The medieval period is characterized as dark age. During this period, superstition and blind following of ancient thinkers prevailed and Aristotelian reasoning and the spirit of scientific inquiry were died out. Thus, no progress occurred in the idea of evolution during medieval period.
With the Renaissance, interest in the natural sciences quickened and the increasing knowledge of many kinds of animals led more and more scientists to consider the concept of evolution favourably. Thus, before Charles Darwin several possible mechanisms have been proposed to account for the evolution; some of them have been considered here as follows.
www.ejbps.com 4 This creationist concept held certain biological theological and philosophical problems example, many fossilized organisms had been discovered that no longer existed on earth, and yet it unbelievable that the perfect products of an omnipotent designer could ever have become obs….. Georges Cuvier (1769-1832), a very influential French biologist and founder of palaentol… offered catastrophic hypothesis as a way out. A series of catastrophes, fires and floods (e.g., N….. Flood) had removed some of the species initially created by God. Apparently useless characters another worry because inferior design could hardly be attributed to a super-intelligent designer explain certain useless characters (e.g., toes of pig), …………(1707-1788) suggested that the Su……….being had created perfectly designed types embodied in the original species, but that new species a from them by a process of hybridization and degeneration. Thus ass was supposed to be a degenerate horse and the ape a degene….. man. But this meant that the assumption of strict fixity of spe…. Had to be relaxed and the concept of nullae speciae novae dip eared from the last revised edition of the book – systema nat…. of ………. Though was not an evolutionist, nor wa……. Greatly concerned with explanations of nature, buthe devoted life industriously in arranging the kinds of animals and pl….. known to him in a convenient system. This system he took to actual and fixed; to discover it and use it was the major tas……. Natural history. He broke the tradition in placing man, “the know……. One, Homo sapiens” in the animal kingdom, the class Mamm……… and the order Primates. Of many categories employed today in classification, it is interesting ……….. used only the kingdom, class, order, genus, and species, but other naturalists soon foun……..
Lamarckism was supported by E. Geoffroy St. Hilaire (1772-1844), Robbert Chambers (1802-1871) and Herbert Spencer (1802-1903), but bitterly opposed by G. Cuvier (1769-1832) and Weismann (1887).
4. Darwinian Period
www.ejbps.com 5 the availability of food supply, but are kept in check by natural diseases, and famine, and man-made wars. Darwin-Wallace theory of natural selection was published in the Journal of the Linnaean Society in July, 1858.
Darwin seemed to agree with Lamarck‟s idea that characters acquired during the life of an organism might be transmissible, in a heredity sense, to its offspring (e.g. Darwin‟s theory of pangenesis). However, it was on the mechanism of evolution that Darwin parted company, with both Lamarck and his own grandfather.
In his book, Origin of Species (fist published in 1859), Darwin replaced the idea of direted variation (brought about by a sentiment interieur) with random variation that was heritable. Regarding the origin of this variation, he was a little vague – he thought that it might be due to environmental influences on the ……… and he was equally vague about the mechanism of inheritance. What he was perfectly clear about was that no matter how the variation originated, it did so withoutany reference to the needs of the organism. Moreover, for Darwin the variation upon which evolution was based involved very small continuous changes away from the original form. He accepted that large abrupt changes might occur, but claimed these were rare and most led to monstrosities. Finally, instead of Lamarck‟s mysterious struggling for perfection, Darwin explained evolution in terms of a completely ordinary and non-mysterious process. Darwin observed that animal and plant breeders had, by the selective breeding of variants, been able to affect considerable changes in the form of domestic plants and animals. This was artificial selection. Similarly, he thought that the struggle between organisms in an overcrowded world by finite resources would ensure that only the fittest survive and this would lead to a kind of natural selection of variants.
www.ejbps.com 6 selection of the fittest, species become represented by individuals which are better adapted.
5. Post-Darwinian Period
The history of evolutionary thought subsequent to the publication of the book „The Origin of Speceis’ may be divided into the following three periods.
(i) The romantic period. The romantic period extended from 1860 to about 1903 and was characterized by extreme enthusiasm for Darwinism, together with un uncritical acceptance of whatever data were claimed to support Darwinism. Negative evidence was given little weight, while illogical extremes of interpretation in order to make observed facts fit Darwinian theory were quite common. Leaders of this group in England included T.H. Huxley (1825-1895), Herbert Spencer (1820-1903) and George Rmanes, while in the United States David Starr Jordan and ASA Gray (1810-1888) were the leaders. As a group, they went to interpretative extremes, reading adaptive significance into every organic structure, even on the most imaginary evidence. They often cited excellent anatomical and taxonomic evidence, but experiments to test adaptive values were unusual.
It should not be thought that these were second-rate biologists who were blinded by the brilliance of a great man like Darwin, on the contrary, they were excellent men in their respective fields. Huxley made splendid contributions to the development of invertebrate zoology, taxonomy and vertebrate anatomy. Spencer was one of the leading philosophers of his time. Romanes started his career as an invertebrate neurologist, but he soon became mainly engrossed in evolutionary problems. Jordan was one of the best ichthyologists of all times. Gray was a botanist of great repute. Nor should it be thought that they never ventured to differ from Darwin, for these men were independent thinkers. Yet the atmosphere of approval was extraordinary.
www.ejbps.com 7 embryologystates that 1. general characters appear before special characters; 2. from the more general characters, less general and finally the special characters develop, 3. an animal during development departs progressively from the forms of other animals; 4. the young stages of animals are like the young (or embryonic) stages of other animals lower in the scale, but not like the adults of those animals. Gegenbaur (1826-1903) was one of the greatest and most influential comparative anatomists. He and his collaborators made exhaustive studies, in complete detail, upon all classes of vertebrates, and used the data so obtained in support of Darwinian theory. Much of the phylogeny (i.e., the evolutionary or anestal history of organisms) of the vertebrates in current textbooks of zoology is taken from the works of Gegenbaur and his collaborators.
Ernst Haeckel (1834-1919) did significant work in anatomy, embryologyand taxonomy. His studies in comparative embryology led him to broaden the principles of Von Baer into the theory recapitulation or biogenetic law, in support of which he published extensively. In one of his writings „Generelle Morphologie‟, Haeckel (1866) defined the biogeneitic law in the following way – an individual organism in its development (……..) tends to recapitualate (or repeat) the stages passed through by its ancestor (phylogeny), on that antogenyrepeats phylogeny.
www.ejbps.com 8 between parts, organs, tissues, etc., of the same individual (Roux; according to Roux, 1881, the struggle is waged among the molecules, tissues and organs; what is at stake is the intake of food which, according to its quality, ensures greater or determinants (now called genes; Weismann).
Thus, according to Weismann, the most important struggle is between the “vital invisible” particles (genes), whether they reside in the body cell (soma) or the sex cells. These determinants are never identical with one another and their ability to “assimilate” food varies from one another. The strongest take the most food, and their descendants prevail over the others. The result is that by the second generation the parts of the organism represented in the ovum by the strongest determinants, are the most highly developed. This explains the dominance of certain characteristics; the determinants of organs favoured by natural selection are better fed than the rest and are the winners in the competition, hence, the strengthening of change in that the rest and are the winners in the competition, hence, the strengthening of change in that direction (Grasse, 1977).
Also prominent during the romantic period was Karl Pearson and Francis Galton who laid the foundation of the new sciences of statistics and biometry which play a prominent role in modern evolutionary studies.
(ii) The agnostic period. The agnostic period extended from 1903 to 1935 and involved a lot of skepticism and disillusionment. Many factors converged to cause this change. One factor was the rediscovery of Mendel‟s laws of heredity in 1900. Today Mendelism is the foundation of most studies in evolution, but then the permanence of the gene seemed to raise fearfulobstacles to the origin of new species. In consequence, genetics was regarded as a sort of blind path at the end of which stood the sign: the gene, the dead end. Another factor was the conclusion which was drawn by Johansen from his hereditary experiment of the size in beans, that selection could be effective only in a stock (or organism) with hereditary variability. Variations produced by the environment (including nutrition, sunlight, temperature and moisture) were unimportant for evolution.
www.ejbps.com 9 Wagner (1868) emphasized the effects of different environments on isolated froms. Gulik pointed out the unavoidability of that an inner directive guides the course of evolution mainly independent of the environment.
Hugo de Vries (1848-1935) in his book The Mutation Theory (1909) distinguished between continuous individual variation and discreate, saltational variations. He applied the term mutation to the latter only. De Vries experimented with the plant evening primrose Oenothera lamarckiana. For De Vries it was these mutations that controlled evolutionary change and which were more important than natural selection itself.
www.ejbps.com 10 Sewal Wright, another pioneer of neo-Darwinism, in 1931 developed a conceptual framework for considering the interaction between genes and their influence on fitness. This is referred to as an adaptive landscape or fitness space. Wright‟s emphasis was on populations of genes. However, R.B. Goldschmidt (1878-1958) who had wide experience in the study of geographic variations, taxonomy and physiological genetics, is the principal proponent of the main alternative to the dominant neo-Darwinian theory.
Punctuated equilibrium concept of evolution. This idea was proposed in 1972 by two American palaentologists, Dr.S.J. gould and Dr.N. Eldredge and has been widely accepted as the best way to interpret evolutionary patterns over geological time (see Henry Gee, 1993). This concept contradicted Darwin‟s idea that species evolve gradually one into other by insensible degree. According to Gould and Eldredge, species evolve by sudden bursts of rapid change that „punctuate‟ long periods of static equilibrium. The punctuated equilibrium concept showed that Darwinian gradualism should have been supported by fossil record, i.e., transitional forms should make up the bulk of fossils. But fossil record shows that species seem to persist with little change over many millions of years. Thus, there is no use to try to find evidence of gradual change in fossil record, because it is not there to begin with (Gee, 1993). According to Gould and Eldredge, species evolve very rapidly, within a few thousand years – too short a time for preservation of the fossil record to be likely. But once evolved species remain unchanged for many times, longer than it makes them to evolve. Thus, stasis (stoppage) of species becomes data of evolution.
www.ejbps.com 11 (iii) The modern synthesis period. The synthesis of a coherent theory of evolution which takes into account all the pertinent facts of modern biology, has been the work of several biologists during the past fifty years. The first edition of Theodosius Dobzhansky‟s now classic book, Genetics and the Origin of Species‟, which appeared in 1937, set the stage, and stimulated biologists in several fields to contribute to the synthetic evolutionary theory. The term modern synthesis was first used by Julian Huxley in 1942. as presented in the famous book – Evolution, The modern Synthesis by Huxley (1942), the modern synthesis or synthetic theory was an amalgamation of the data and concepts of genetics taxonomy, embryology, biogeography, and other disciplines (Stanley, 1979). Likewise books by zoologist Ernst Mayr (1942, 1963) showed how the modern theory could explain the origin of variation patterns in higher animals and G,L.Stebins (1950) attempted to do the same for higher plants. E.B. Babcock‟s (1947) study of the genus Crepis, carried over a period of more than thirty years, is one of the most thorough and comprehensive studies ever made upon a single genus of plants and it is one of the major supports of the neo-Darwinian theory. Clausen and coworkers in 1951 have published extensive studies on the behaviour of plants when grown in widely different habitats. A leading palaeontologist G.G. Simpson showed in his two books – Tempo and Mode in Evolution (144) and The Major Features of evolution (1953) that the fossil record of higher animals is best explained by assuming that throughout the evolutionary history of living things those same processes took place which were being experimentally demonstrated by many workers in populations of contemporary animals. In 1960, Bernhard Rensch has made a strong case for the belief that the evolution of genera families and higher categories of animals has taken place through an extension into long periods of time of those same processes which at any one time level govern the origin of races and species.
www.ejbps.com 12 Theories of evolutionary changes (E.Mayr, 1970).
A. Monistic (single – factor explanations)
1. Ectogenetic: Change directly induced by the environment. a. Random response (for example radiation effects)
b. Adaptive response (Geoffroyism)
2. Endogenetic: Changes controlled by intrinsic forces a. Finalistic (orthogenetic)
b. Volitional (genuine Lamarckism) c. Mutational limitations
d. Epigenetic limitations. 3. Random events (“accidents”) a. Spontaneous mutations b. Recombination
4. Natural selection
B. Synthetic (multiple-factor explanantions) 1b + 2a + 2b = most “Lamarckian-type” theories 1b + 2b + 2c = some recent “Lamarckian” theories
1b + 3 + 4 = Late Darwinian, most non-mutationists during first three decades of 20th
century
3 + 4 = early “modern synthesis”
1a + 2c + 2d + 3 + 4 = recent “modern synthesis”
The modern synthetic theory selected the best aspects of the earlier hypotheses and combined them in a new a original manner. In essence a two-factor theory, it regards the diversity and harmonious adaptation of the organic world as the result of a steady production of variation and of the selective effects of the environment.
www.ejbps.com 13 a balance of conflicting forces. The current theory of evolution the “modern synthesis”, owes more to Darwin than to any other evolutionist and is built around Darwin‟s essential concepts. Yet it incorporates much that is distinctly post-Darwinian. The concepts of mutation, variation, population, inheritance, isolation, and species, all of which were quite nebulous in Darwin‟s days, but now are much that is distinctly post-Darwinian. The concepts of mutatin, variation, population inheritance, isolation, and species, all of which were quite nebulous in Darwin‟s days, but now ae much better defined and understood, have been incorporated in recent “modern synthesis” (Mayr, 1970).
PRESENT STATE OF EVOLUTIN IDEA
The current synthetic theory of evolution recognizes five basic types of processes – Gene mutation, Changes in chromosome number and structure, Genetic recombination, Natural selection, and Reproductive isolation. The first three provide the genetic variability without which change cannot take place; natural selection and reproductive isolation guide populations of organisms into adaptive channels. In addition, three accessory processes affect the working of these five basic processes. Migration of individuals from one population to another as well as Hybridization between races or closely related species both increase the amount of genetic variability available to a population. The effects of chance, acting on small populations, may alter the way in which natural selction guides the course of evolution.
Revision questions
1. Discuss critically the following statement:
„Evolution was born in England, but found its home in Germany‟.
2. Discuss the main features of post-Darwinian romantic period and agnostic period.
3. Discuss how did the idea of evolution mature during the 19th century?
4. What is the theory of modern synthesis? Describe its origin growth and essence. 5. Discuss the main points of pre-Darwinian period.
6. Describe the evolutionary ideas of early Greek philosophers. 7. Write short notes on the following:
(i) Father of family trees; (ii) Argument of design
www.ejbps.com 14 (v) Weismann‟s determinants.
Theories of organic evolution
Two explanations have been offered for the origin of the different forms of life – special creation and evolution. Many evidences have been presented in previous chapters to support the concept of evolution, and almost without exception modern biologists are convinced of evolution. Uptill now, the following convincing theories have been forwarded to explain the mechanism of evolution. The first of these theories, which is now only of historical singnificance, was that of Jean Haptiste de Lamarck (1744-1829).
Theory of inheritance of acquired characters (Lamarckism)
The theory of inheritance of acquired characters states that modifications which the organism acquires in adaptation to the environments which it meets during its lifetime are automatically handled down to its descendants and so become part of heredity. This theory was propounded by a renowned French……… ……….Revolution. Lamarck spent the early part of his life as a botanist. Then at the age of 50 he turned his attention to zoology, particularly to the study of invertebrates (The terms “invertebrate” and “biology” have been coined by Lamarck). As a result of his systematic studies he became convinced that species were not constant but rather were derived from pre-existing species. This idea was in total conflict with the view of the period – that of fixity of species. As a result Lamarck‟s views were challenged by most of the biologists of that time, particularly by Georges Cuvier. In 1809, Lamarck published philosophie Zoologique, which included his theory explaining the changes that occur in the formation of new types. Although, his views on evolutionary mechanism are outmoded now, he still occupies a very important place in the history of evolutionary thought. He was the first evolutionist to conclude that evolution is a general fact covering all forms of life. His evolutionary ideas can be discussed in brief as follows.
1. Internal forces of life tend to increase the size of the organism. New structures appear because of an “inner want” of the organism, i.e., the internal forces of life tend to increase continuously the size of an organism and its component parts.
www.ejbps.com 15 3. Use and disuse. The various organs became greatly improved through use or reduced to vestiges through disuse.
4. Inheritance of acquired characteristics. Such bodily modifications, in some manner, could be transferred and impressed on the germ cells to affect future generation. Thus, inheritance was viewed by Lamarck simply as the direct transmission of those superficial bodily changes that arose within the lifetime of the individual owing to use or disuse (Volpe, 1985).
Thus, Lamarck believed that organic changes seen in animals were resulted by the influence of environment on the gradual changes of species due to their tendency to become more and more perfoect. According to him, when an animal‟s environment changes, its needs change, and this leads to special demands on certain organs. Organs used more extensively would enlarge and become more efficient. Conversely, an organ or organs, no longer used, would degenerate and atrophy. He postulated that such changed characteristics (acquired traits) would be transmitted to the offspring.
Examples of Lamarckism
1. The deer-like ancestor of giraffe lived in places (Africa) where the ground was almost invariably parched and without grass. Obliged to browse upon trees, it was continually forced to stretch upwards. This habit maintained over long periods of time by every individual of the race had resulted in the forelimbs becoming longer than the ……. Ones, and neck so elongated that a modern giraffe can raise his head to a height of eighteen fet without taking his forelimbs off the ground.
2. Ducks and other aquatic birds invaded waters from land in search of enough food, because food was scarce on land and these birds did not had power to fly. In water, the duck would stretch its toes apart to give more push during swimming. This new characteristic would be inherited, and the subsequent generation of duck would upon stretching their toes form a more defined web. Each generation would do the same, until the webbed foot seen on ducks today was fully formed. This would then be passed on from generation to generation, essentially unchanged once the perfected state was attained.
www.ejbps.com 16 (lying towards bottom) migrated towards upper side and, thus, both eyes are on one side of the body.
4. The whales lost their hindlimbs as the consequence of the inherited effect of disuse.
5. The wading birds (e.g., Jacana) developed its long legs through generations of sustained stretching to keep the body above the water level.
6. Snakes have elongated body accompanied by loss of limbs. The continuous creeping through holes and crevices made limbs continuously useless for locomotion with the result that limbs become completely lost in snakes.
7. Eyes are reduced in moles since they live under ground. In cave animasl also, eyes might become functionless and might even disappear.
8. For plants Lamarck accepted the theory of his compatriot St.Helaire, who developed the notion that plant form is shaped by the combined effects of the environment.
There are other effects such as vestigial organs in living animals due to disuse. Claws in Carnivora, sensitive skin and tactile points on the ventral side of the body and callosities of plam in hard workers, exemplifying Lamarckian theory.
Significance of Lamarckism. Lamarckian theory was simple and it had some appeal, as it provided a way in which changes in organisms could come about. It was the first completely comprehensive mechanistic theory that was offered. Furthermore, it was the theory that lent itself to predictions and, therefore, to testing. Thus, Lamarckian theory enjoyed popular acceptance for near about 70 years, because it was exemplified by many common examples. Most persons know that exercise results in larger muscles.
Critical Analysis of Lamarck’s Propositions
Lamarck defended his evolutionary theory vigorously until his death. For it, he suffered both social and scientific ostracism, but he had the courage of his convictions. However, he was criticized for the following reasons by the contemporary scientists during his lifetime and afterwards.
www.ejbps.com 17 increase in size, there are many cases, where evolution proceeded not only without any increase in size but rather through a reduction in size. Many plants contradict this Lamarckian principle by showing such a reduction in size during their evolution. Many ferns and conifers which became extinct were gigantic trees and the more highly evolved flowering plants are really much smaller in size.
2. The second Lamarckian principle that new organs result from new needs, is quite manifestly false. In the case of plants, Lamarck believed that the environment acted directly upon the plant causing the production of such new characters as might adapt the plant to its environment. Acted through the nervous systems; in other words, the desire of the animal leads to the formation of new structures. In its crudest form this would mean that the man who mused “Birds can fly, so why can’t I ? should have sprouted wings and taken to the air.
3. The third Lamarckian principle that organs will develop due to use and degenerate due to disuse, may be correct as far as growth of an organ within the lifetime of an individual is concerned. For example, it is a commonly observed fact that if muscles are put to use these would develop. Hwoever, this principle is meaningful only when it is studied in relation to the following fourth principle.
4. The fourth and final proposition of Lamarck was that the inheritance of characters acquired during the lifetime of the individual. This principle has been tested by many biologists who have devised many types of experiments for it and have found it entirely incorrect. Certain experiments which have discredited it are the following. 1. The noted German scientist August Weismann was the first person who for the first time made a definite distinction between heritable changes and those which cannot be inherited. In 1890, he performed some experiments to test if characters may disappear due to disuse. This he did by cutting the tails (mutilation) of white mice for more than 20 generations to see if this has any effect on tail length. The measuring of tail length of the offsprings of 20 successive generations, revealed that on average, the tails were not shorter. It means that, acquired character (cut tail) was not inherited.
www.ejbps.com 18 from this pair were black. This shows that the environment does not affect the heredity as has been suggested by Lamarck.
3. Loeb artificially fertilized the eggs of sea urchin by certain chemical stimuli and these parthenogenetically fertilized eggs produced the generations. The members of which possessed no parthenogenesis.
4. Boaring of ears and nostrils in Indian women has been continued as the tradition from centuries but their offsprings do not show any trace of holes in ears and nostrils. 5. The Chinese bound the feet of their women for many generations, yet this has not resulted in any modification of the feet of present-day Chinese women.
6. Jewish boys and also Muslim boys) have been circumcised for thousands of years, but this has not resulted in a tendency toward the reduction of the prepuce in them. All the aforesaid cases and experiments clearly showed that Lamarckian theory is not tenable.
Neo-Lamarckism
www.ejbps.com 19 believed in the interplay of structure, function and environment as the whole truth of evolution. However, no evolutionist of today is adherent to neo-Lamarckism.
The following are the evidences of transmission of acquired characters.
1. McDougall conducted learning experiments in rats and from the results obtained he tried to suggest that learning as an acquired character can be inherited. During his experiments, the rats were dropped into a tank of water from which there were two exits, one lighted and one dark, but no always the same one. A rat leaving by the lighted exit received an electric shock, while one leaving by the dark exit received no shock. Thus, the number of trails required fro a particular rat to learn always to select the dark exit constituted a measure of the speed of learning. These rats were then bred and their descendants were similarly studied. It appeared that the speed of learning increased from generation to generation and, so, McDougall concluded that learning, an acquired trait par excellence, is inherited.
Some serious criticisms have been raised against McDougall‟s experiments. It is suggested that genetic constitution of rats was not controlled. Moreover, in the progeny of control rats not subjected to these learning tests, the learning habit was found to change the same way as in the treatments. So, it was said that some unanalyzed changes in the technique may be responsible for the recorded increase in the speed of learning. Lastly, the most strange aspect about McDougall‟s experiment is, that when the same experiments, when repeated in other laboratories, never gave similar results.
2. F.B. Summer reared the white mice at 20°C to 30°C, because of which they developed longer bodies, tail and feet. When these mice were bred at lower temperatures their offsprings showed their normal proportions of size.
www.ejbps.com 20 4. Guver and Smith induced the hereditary changes in the eyes of fetuses of rabbit by simply destroying lens of living female with a needle in situ. The antilens serum has been produced in the blood of these animals.
5. Gr…….. and Detie….. reared rats in cages placed on rotating table for several months. Consequently, they became adapted to the rotating condition to such an extent that when rotation was stopped, they showed signs of ………….(dizziness) and other physiological conditions. This condition was inherited for several generations. 6. Various acquired diseases described by Brown Sequard are inherited from generation to generation. For example, exophthalmia was caused in parents by injuring in brain the restiform body. This disease was, later on, inherited to several generations. The other diseases such ………. And ……….. are produced by injury to the restiform body near the rib of calamus. Later, these were found to be inherited.
7. Kammerer worked on the tailed amphibian proteus anguinus which lives in complete darkness in the water of underground cave. It was blind and colourless. He brought proteus in daylight, due to which it became coloured (brown and blacm), which passed on to its progeny. Eyes of Proteus also developed normally in daylight. This showed that any change in environmental conditions, induced changes in the animals and the acquired characters were inherited.
Further, Winterbert (1962) has tried, with some success, to give Lamarckism a chemical basis by involving the capacity of everyliving being to react to environmental changes and aggressions. According to Winterbert, nothing is acquired by the living organism that is not the response of internal factors to an outside influence or ethological change. Living means reacting, never undergoing. Above all, it means not waiting around for a fortunate chance occurrence to save the situation (Grasse, 1977).
2. Theory of Natural Selection (Darwinism)
www.ejbps.com 21 with such revolutionary ideas as Newton‟s law of gravitation and Einstein‟s theory of relativity (Volpe, 1985). The concept of natural selection was explained clearly and convincingly by Darwin in his masterpiece – The Origin of Species (The full title of the book was On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life). This epoch-making book was the fruition of more than twenty years of meticulous accumulation and analysis of facts. The first edition of the book, some 1,250 copies, was sold out on the very day it appeared, November 24, 1859. This book had opened a Pandora‟s box; it was immediately both acidly attacked and effusively praised.
Charles Robert Darwin was born on February 12, 1809 in Shrewsbury, England. His personality was not the type that one associates with a successful man, let alone a greatone. While his family held a distinguished reputation (his grandfather Erasmus Darwin was a noted poet scientist, his father Robert, was an eminent physician, and his mother was Wedgwood, the family of pottery fame), Charles was a singularly undistinguished individual whose contemporaries viewed him as just a normal, gentle, cautious, morally upright English lad. After completing his early education at Shrewsbury, Charles Darwin was sent at the age of fifteen to study medicine at the University of Edinburg. His medical interests soon dissipated, and he became something of an academic rover, studying a little science, but finding it too formal for his tastes.
His, father then suggested him to become a clergyman, which appealed him. So, he got transferred, after two years, to Christ‟s college, Cambridge University, to study theology. There too he spent his energy in card-playing and drinking. He was obviously not ready for clerical commitments.
www.ejbps.com 22 The ship Beagle left Plymouth on December 27, 1831 and visited many islands of the Atlantic ocean, some coasts of South America and some islands of south Pacific, of which Galapagos Islands are the most important. During his five years (from 1831 to 1836) on the Beagle, Darwin recorded almost everything he observed and sent an enormous amount of material back to England.
Upon his return home in 1836, he spent two years writing a book of his experiences, The Voyage of the Beagle. In 1838, he got married and later had two daughters and five sons. From 1838 to 1841. Darwin acted as the secretary of geological society, where he came in contact with an eminent geologist, Charles Lyell. It was not until 1844 that Darwin developed his idea of natural selection in an essay, but not for publication. He showed the manuscript to ……… who encouraged him to prepare a book. Darwin still took no steps toward publishing his views. Perhaps he was well aware of how Lamarck‟s earlier theory had been received and water all possible supporting evidence before publishing his own new theory. It appears that Darwin might not have prepared his famous volume had not a fellow naturalist in the Dutch East Indies, Alfred Russel Wallae (1823-1913), independently conceived of the idea of natural selection. Wallace had traveled widely in tropical South America and Sourheastern Asia. For studying the flora and fauna of these regions. Walla…. Also inspired by reading Malthus‟s essay, and the idea of natural selection came to him in a flash of insight during a sudden fit of malarial fever (this happened in February, 1858, when he was workin gin the island of Ternate in Indonesia). In June of 1858, Wallace sent Darwin a short essay “On the tendency of varieties to depart indefinitely from the original type”, and asked him if he thought of sufficient interest to present it to the Linnaean Society. We can easily imagine Darwin‟s amazement upon receiving Wallace‟s essay. Upon the insistence of the geologist Charles Lvell and the botanist Joseph Hooker, Darwin prepared an abstract of his conclusions for joint publication with Wallace‟s essay. Wallace‟s essay and a portion of Darwin‟s manuscript, each containing remarkably similar views, were read simultaneously before the Linnaean Society in London on July 1, 1858. The joint reading of the papers stirred little interest.
www.ejbps.com 23 honour of establishing the mechanism by which evolution is brought about, but it was the monumental The Origin of Species with its impressive weight of evidence and argument that left its mark on mankind. The fact that this book, written by Darwin alone, exposed the public at large to the theory of natural selection, and that Wallace was still in Indonesia at the time of the controversy and could not champion Darwin‟s defence by adding his own views, brought the public to associate only Darwin with this theory. Thus the theory is called the Darwinian, rather than the Darwin-Wallace theory of evolution.
Darwin died on April 19, 1882, when he was 73 years old. During his life, he write numerous books, journals, etc. some of them can be listed in Table (5-1).
Table 5-1 Some publications of Charles Darwin.
Title of book Year of Publication
1. Journal of researches
2. The structure and distribution of coral reefs 3. Geological observations on South America
4. A monograph on Cirripedia (Barancles, living and fossil, 4 volumes) 5. Origin of species
6. The fertilization of orchids
7. The variation of plants and animals under domestication 8. The descent of man
9. The expression of the emotions in men and animals 10. insectivorous plants
11. The effects of cross and self-fertilization in the vegetable kingdom 12. different forms of flowers on plants of the same species
13. The power of movement in plants
14. The formation of vegetable mould through the action of worms
1839 1844 1846 1851, 1854 1859 1862 1868 1871 1872 1875 1876 1877 1880 1881 Facts that influenced Darwin’s Thoughts
During the period in which Darwin massed his evidence and developed his natural selection theory, many things affected his thinking, of which three, in particular, deserve mention here.
www.ejbps.com 24 quakes, volcanoes, and other natural forces. Darwin was impressed by Lyell‟s emphasis on the great antiquity of the earth‟s rocks, and gradually came to conclusion that the characteristics of organism as well as the face of the earth could change over a vast span of time.
2. In the late 1830s, Darwin attended the meetings of animal breeders and intently read their publication. Animal breeders were conversant with the variability in their pet animals, and dwelled on the technique of artificial selection. Thus, the breeders selected and perpetuated those variant types that interested them or seemed useful to them. The breeders, however, had only vague notions as to the origin, or inheritance, of the variable traits.
3. Having explained the origin of variation (although incorrectly), Darwin wondered how artificial selection (a term familiar then only to animal breeders) could be carried in nature. There was no breeder in nature to pick and choose. In 1838, Darwin found the solution in Thomas Robert Malthus book „An Essay on the Principle of Population‟ in which Malthus asserted that the reproductive capacity of mankind far exceeds the food supply available to nourish an expanding human population. Humans compete among themselves for the necessities of life.
It, thus, occurred to Darwin that competition existed among all living things. Darwin then envisioned that the “Struggle for existence” might be the means by which the well-adapted individuals survive and ill-adjusted are eliminated. Darwin was the first to realize that perpetual selection existed in nature in the form of natural selection. In natural selection, as contrasted to artificial selection, the animal breeder or horticulturist is replaced by the conditions of the environment that prevent the survival and reproduction of certain individuals. Thus, natural selection is a term serving to inform us that some individuals leave more offspring than others. It is not purposeful or guided by a specific aim; it does not seek to attain a specific end.
Theory of Natural Selection
www.ejbps.com 25 Thus, natural selection is a continuous process of trial and error on a gigantic scale, for all of living matter is involved. It includes the following elements.
1. The universal occurrence of variation. Variation is the characteristic of every group of animals and plants and there are many ways in which organisms may differ. (Darwin and Wallace did not understand the cause of variation, and assumed it was one of the innate properties of living things. We now know that inherited variations are caused by mutations.)
2. An excessive natural rate of multiplication. Every species, in the absence of environmental checks, tends to increase in a geometrical manner. If a population of a given species doubles in one year and if there are no checks on its increase, it will quadruple the next year, and so on. Such a great reproductive potential of different species may be easily observed in nature. It has been estimated that a common Atlantic coast oyster may shed as many as 80 million eggs in one season. A salmaon produces 28,000,000 eggs in a season. A single pair of English sparrows would be the ancestors of over 275 billion individuals in 10 years if they and their descendants could reproduce at their natural rate without any check. Darwin calculated that even a pair of elephants which are about the slowest breeding animals known, could in the absence of any checks, have 29 million descendants at the end of 800 years.
Thus, more organisms of each kind are born than can possibly obtain food and survive. Since the number of each species remains fairly constant under natural conditions fairly constant under natural conditions, it must be assumed that most of the offsprings in each generation perish. If all the offsprings in each generation perish. If all the offsprings of any species remained alive and reproduced they would soon crowd all other species from the earth.
3. Struggle for existence. Since more individuals are born than can survive there is an intraspecific or interspecific or environmentals are born than can survive there is an intraspecific or interspecific or environmental struggle for survival, a competition for food, mates and space. This contest maybe an active kill-or-be-killed struggle or one less immediately apparent but no less real, such as the struggle of plants or animals to survive drought or cold.
www.ejbps.com 26 their possessors. This idea of “The survival of the fittest” is the core of the theory of natural selection.
5. The inheritance of the mutations or recombinations that make for success in the struggle for existence. The surviving individuals will give rise to the next generation and, in this way, the “successful” variations are transmitted to the succeeding generations. The less fit will tend to be eliminated before they have reproduced.
Successive generations in this way tend to become better adapted to their environment; as the environment changes further adaptations occur. The operation of natural selection over many generations may produce descendants which are quite different from their ancestors, different enough to be separate species. Furthermore, certain members of a population with one group of variations may become adapted in a different way, or become adapted to a different environment. In this way two or more species may arise from a single ancestral stock.
Critical Analysis of Darwinism
www.ejbps.com 27 Certain other demerits of Darwinism. Besides its numerous merits, Darwinism has following demerits.
1. Darwinism or natural selection theory does not account for the beginning of organs, which may appear at first as the veriest rudiments having as yet no selection value. In other words, it remains concerned with the survival of the fittest, but not for the arrival of the fittest. Thus, to give rise to such specializations as elaborate mimicry, or the electric organ of the torpedo, etc., which are of apparent advantage only in the perfected state, natural selection, acting only upon minute gradations toward perfection, seems inadequate. The same is true of so complex a specialization as the eye and its function in the vertebrates or in the insects and crustaceans.
2. Over-specialization in certain cases like extinct Irish deer in which huge antlers outweigh the entire skeleton, or the immense spiral tusks of the Jefferson mammoth, or the minute fidelity of certain mimicking insects such as Kallima, or huge dinosaurs of Mesozoic – all cannot be explained on the basis of continuous variations and natural selection. These organs or body structures should not have reached such a harmful stage, if natural selection was operating. However, such cases of overspecializations have been explained by Darwin on the basis of discontinuous variations or “sports” which, according to him, do not play any role in evolution. 3. Natural selection cannot account for degeneracy. To say an organ is no longer useful and, hence, disappears, is to state the effect and not the cause. If under changed conditions a character built up by natural selection becomes a menace, the reversal of selection can accomplish its removal but this will not suffice where the characteristic is an indifferent one.
4. One of the classical objections to natural selection is that new variations would be lost by “dilution” as the individuals possessing them bred with others without them. We now know that although the phenotypic expression of a gene may be altered when it exists in combination with certain other genes, yet the gene itself is not altered and is transmitted to succeeding generations.
5. Darwin indirectly accepted the Lamarckian idea of inheritance of acquired characters in the form of pangenesis hypothesis, which cannot be accepted in the light of knowledge of genetics made available in the present century.
www.ejbps.com 28 involving blood transfusions and later, transplants of ovaries between black and white varieties of rabbits and chickens. Gametes produced by the transplanted ovaries were consistent with the phenotype of the individual in which the ovary originated and not with the animal currently carrying the ovary. Blood transfusions had no effect on the gametes produced. These experiments readily demonstrated that the pangenesis hypothesis was incorrect.
6. Lastly, some persons have objected natural selection because it is essentially a materialistic doctrine, depending as it does purely on the laws of chance.
Neo-Darwinism
Neo-Darwinism is a modified from of Darwinism. The Neo-Darwinians like T.H. Huxley and Herbert Spencer of England, D.S. Jordan and Asa Gray of United States, and E. Haeckel and A. Weismann of Germany believed that natural selection has accounted everything that is involved in evolution. In their dogmatic belief they went further to secure enough proof towards natural selection.
Certain Neo-Darwinians, such as Weismann and his followers rejected Darwin‟s theory except its principal element of natural selection. These Neo-darwinians, though distinguished between germplasm and somatoplasm of living organisms in their germplasm theory, yet they could not appreciate the role of mutations in evolution. While Darwin believed that the adaptations result mainly by a single source, i.e., natural selection, Neo-Darwinians thought that adaptations result from multiple forces and natural selections is only one of these many forces. Neo-Darwinians also believed that characters are not inherited as such but there are character determiners, the determinants or biophores, which control only the development. The ultimate charcter would result out due to the interaction of the determiners, activity of the organism and the environment during development. Thus, Neo-Darwinism was incomplete and partly wrong because it lacked present understanding of genetics.
Maturation of Neo-Darwinism into Modern Synthesis
During later half of 19th century and at the beginning of 20th century, many important
www.ejbps.com 29 1900 made it clear that (1) the factors given to the offspring by the parents do not “mix” but are segregated, and (2) in more than one pair of contrasting characters are considered in the same cross, the factors responsible for these are inherited independently. Likewise, mutation theory of evolution of Hugo de Vries (1886, 1887) stated that new species arise by sudden changes or steps called mutations rather than by gradual processes. According to de Vries, it was mutations and not selection that should be considered as the primary factor in evolution. Further, the studies of Wagner (1868) suggested the role of geographic or spatial isolation in the formation of every species, race, or tribe of animals or plants on the earth. Even more, certain population geneticists realized that the actual physical struggle between animals for survival or the competition between plants for space, sun and water is much less important as an evolutionary force than Darwin believed. The evolution of any given kid of organism occurs over many generations during which individuals are born and die, but the population has a certain continuity. Thus, the unit in evolution is recognized not the individual but rather a population of individuals.
www.ejbps.com 30 All these modern understandings in cytology, genetics, cytogenetics, population genetics, and evolution gave away for the formulation of a coherent theory called modern synthesis around 1930s, by S.Wright, H.D. Muller, Th.Dobzhansky, R.B. Goldschmidt, J.S. Huxley, R.A. Fisher, J.B.S. Haldane, Ernst Mayr and G.I. Stebbins. 3. MODERN SYNTHETIC THEORY
The modern synthetic theory of evolution involves five basic processes – mutations, variations, heredity, natural selection and isolation. In addition, three accessory processes affect the working of these five basic processes. Migration of individuals from one population to another as well as hybridization between races or closely related species both increase the amount of genetic variability available to a population. The effect of chance acting on small populations, may alter the way in which natural selection guides the course of evolution (Stebbins, 1971).
1. Mutation. Alteration in the chemistry of gene (DNA) is able to change its phenotypic effect (i.e., nature of polypeptide) is called point mutation or gene mutation. Mutation can produce drastic changes or can remain in significant. There are equal chances of a gene to mutate back to normal. Most of the mutations are harmful or deleterious and lethal but not all. Most of the mutant genes are receissive to normal gene and these are able to express phenotypically only in homozygous condition. Thus, point mutations tend to produce variations in the offspring.
2. Variation (Recombination). The nature of genetic variations caused by reshuffling of genes during sexual reproduction (recombination) was very little known at the time of Darwin. Recombination – that is, new genotypes from already existing genes – is of several kinds: 1. the production of gene combinations containing in the same individual two different alleles of the same gene, or the production of heterozygous individuals (meiosis); (2) the random mixing of chromosomes from two parents to produce a new individual (sexual reproduction); (3) the mixing of a particular allele with a series of genes not previously associated with it, by an exchange between chromosomal mutations such as polyploidy, deletion, duplication, inversion and translocation also result in variation.
www.ejbps.com 31 other environment that is open to it, are favoured in the struggle for existence. As a result, the offsprings are able to benefit from the advantageous characterstics of their parents.
4. Natural selection. Natural selection brings about evolutionary change by favouring differential reproduction of genes. Differential reproduction of genes produces change in gene frequency from one generation to the next. Natural selection does not produce genetic change, but once genetic change has occurred it acts to encourage some genes over others. Further, natural selection creates new adaptive relations between population and environment, by favouring some gene combinations, rejecting others and constantly moulding and modifying the gene pool.
The workings of natural selection are exceedingly complex because of the range of organizational levels at which it functions. Selection discriminates among available reproducible biotic entities to produce more efficiently adapted units. Natural selection operates upon every stage in the life history of an organism it produces non-random differential reproduction of biological units and may affect any biotic entity from the molecular to the community level. Examples of levels at which natural selection makes differential discrimination are the following : intermolecule, intergene, interchromosome, intergamete, interindividual (Darwinian selection), interdemic, interracial, interspecific and interecommunity. Darwinian selection may result from differential natality among others.
5. Isolation. Isolation of organisms of a species into several populations or groups under psychic, physiological or geographical factors is supposed to be one of the most significant factors responsible for evolution. Geographical isolation includes physical barriers such as high mountains, rivers, oceans and long distances preventing interbreeding between related organisms. Physiological ……….. help in maintaining the individuality of the species, since these isolations do not allow the interbreeding amongst the organisms of different species. This is called reproductive isolation.
www.ejbps.com 32 These populations, thus, become different from each other morphologically and genetically, and they become reproductively isolated, forming new species.
4. WEISMANN’S GERM PLASM THEORY
AUGUST Weismann (1834-1914) was the German neo-Darwinian biologist who proposed the germ plasm theory which was published in the book Das Keimplasma. He asserted the continuityof germ plasm as the main criterion for inheritance of characters. All the heritable variations have their origin in germ cells and a new type of organisms arise only from changed type of germ cells. Thus, Weismann‟s germ plasm theory rejected outrightly the Lamarckian concept of inheritance of acquired characters and Darwin‟s pangenesis hypothesis. While Darwin‟s pangenesis hypothesisis centripetal in nature, germ plasm theory is centrifugal. The main points of germ plasm theory are the following.
1. Existence of somatoplasm and germ plasm. All substances a of an organism can be divided into two parts, the germ plasm (which is theprotoplasm of germ cells such as sperms and ova) and somatoplasm (which is the protoplasm of somatic or body cells). The germ plasm is thought to be the actual vehicle of heredity; it is not affected by any influence either from the body or the external environment. The somatoplasm is thought not to play any role in heredity.
www.ejbps.com 33 (somatoplasm) is mortal, it perishes with the death of an organism. Germ plasm can produce the somatoplasm but somatoplasm cannot produce the germ plasm
3. Architecture of germ plasm (Concept of determinants). In 1904, Weismann proposed that every distinct part of an organism is represented in the sex cell by a separate particle – the idioplasm or determinant. Each determinant is supposed to be made up of still smaller units called biophores. The sum total of determinants would represent the parts of the adult organism with all their peculiarities. The complete set of determinants would be handed down from generation to generation, which would account for hereditary transmission of characters. The determinants, according to Weismann, are localized in the chromosomes of the nucleus, just as are the genes of modern genetics.
Further, during the cleavage of the egg, the various determinants become segregated into different cleavage cells or blastomeres. Eventually each blastomere would contain only determinants of one kind and it would be forced to develop or differentiate in a specific way in accordance with the determinants present in it. Only the cells having the sex cells among their descendants tend to preserve the complete set of determinants, since these would be necessary for directing the development of next generation.
4. Paralled induction. In apposition of Lamarck‟s inheritance of acquired characters, Weismann had introduced the idea of paralled induction. According to this concept, the stimulus affects simultaneously the germ plasm and the body (soma). He proposed the occurrence of an internal stimulus which affects germ cells and results in heritable variations. The stimulus, according to Weismann, is the nourishment which is necessary for determinants and biophores (since they are living units). Those determinants or particles which obtain better nourishment are fast-growing and stronger than those getting less nourishment. Correspondingly these particles tend to produce either strong or weaker part or organ in organisms. Thus, Weismann assumed a struggle for existence between better nourished determinants, and there lies the causes of appearance and disappearance of variations.
Objections to Weismann’s Germ plasm Theory
www.ejbps.com 34 during cleavage and for its failure for explaining causes of regeneration and asexual reproduction.
Significance of Weismann’s Germ plasm Theory
1. It made ground for the understanding of the concept of particulate inheritance of Mendel.
2. It provides some clue about genes (determinants) which reside in chromosomes and represent some part of animal body.
3. According to the embryologists, the greatest contribution of this theory is that it proposes the division of germ plasm and somatoplasm during cleavage of the zygote during the embryogenesis (e.g., cleavage of Ascaris).
4. The idea of continuity and immortality of germ plasm prepares the ground for the continuity of chromosome or DNA from one to next generation.
5. MUTATION THEORY
Darwin recognized two types of variations in nature, viz., 1. Minor and continuous and 2. major and discontinuous ones. He considered minor and continuous variations important in the origin of new species. Darwin considered major variations quite insignificant and for them he coined the terms sports or salutatory (L. saltare = to leap) variations. In England, William Bateson collected a large number of species showing discontinuous or salutatory variations. In Holland, Hugo de Vries (1901) gave much importance to these variations and proposed that new species arise not by the accumulation of minor and continuous variations through natural selection, but by the suddenly appearing salutatory variations for which he coined the term mutation (L.mutare = to change). De Vrie‟s mutation theory states that species have not arisen through gradual selection accumulated for hundreds or thousands of years but have appeared by sudden jumps (saltations) and transformation.
www.ejbps.com 35 spontaneous changes in some of these wild plants. These plants differed considerably in stem height, flower colour andleave‟s shapes. He observed that these changes were heritable and ultimately led to several new varieties. He succeeded in cultivating all these new varieties and named them as mutant varieties. In fact, he selected for his breeding experiments two mutant varieties –Oenothera laevifolia, characterized by smooth leaves and O. brevistylis characterized by short styles. And he observed that these features were breeding true and so, he regarded these mutant strains as the distinct species. Thus, de Vries recognized thefollowing seven distinct species of Oeonothera.
1. O.gigas. Giant and stout plants with large flowers and deep green leaves. 2. O.rubrinervis. Fruit red veined, leaves pale green and stem slender and brittle. 3. O.oblonga. dwarf and weak plants with oblong leaves.
4. O. albida. Weak plants with whitish-pale leaves.
5. O.laevifolia. Leaves narrow and smooth and pale flowers.
6. O.brevistylis. Round leaves, flowers with very short style and flattened stigma. 7. O. vanilla. Very short or dwarf variety (20-30 cm); leaves sessile.
Of these varieties which arose suddenly from the parental variety O. lamarckiana, the O.gigas and O.rubrinervis were progressive varieties; the O.laevifolia, O.brevistylis and O. vanniella were retrograde varieties and O. oblonga and O. albida were weak and inconstant forms and could be grown only under strict protection.
Characteristics of Mutation theory
De Vries’s mutation theory has the following characteristics.
1. Mutations appear from time to time among the organisms of a naturally breeding species or populations. The organisms with mutations are called mutants. These mutants are clearly distinct from their parents.
2. Mutations are heritable and form new species. They do not disappear by crossing. 3. Mutations arise suddenly in one step, i.e., new species arise suddenly in one step and not gradually.
4. Mutations occur in all possible directions and may be advantageous or disadvantageous.
